A system applying decentralized control paradigms can easily reach several local optimal solutions, while it is hard for such a system to check which solution is the global optimal solution. The systems are sometimes trapped in locally optimized situations, and cannot get out without outside interferences. The “circular mill” of army ants is a typical example for the local-optimization issue. For army ants that are blind and move by following the ant ahead, an isolated group of ants may form a circle which will get larger and larger until the ants die of starvation.
(picture from T. C. Schneirla. Army ants. A study in social organization. W. H. Freeman & Co, San Francisco, 1971.).